1. Field of the Invention
The present invention relates to autostereoscopic displays, made mostly in the form of flat panels for use in televisions.
2. Description of the Related Art
Image resolution is one of the most important characteristics in electronic imaging. However, when conventional methods of autostereoscopic imaging is applied to flat panel displays, the horizontal resolution suffers.
Conventional methods employ parallax barrier or lenticular lenses or an array of light stripes as in the backlight of a liquid crystal display (LCD) panel to separate viewing zones of the left and right images of a stereo-pair, thus providing selective viewing of the left and right images by the right and left eyes of a viewer.
Conventionally, two images of a stereo-pair are displayed on the same image panel but on different sets of columns, for instance, a left image on the odd columns and a right image on the even columns. Since both left and right images are displayed in the same panel, resolution of each image is less than half the overall panel resolution.
There are two different ways to achieve the same resolution of stereoscopic images as the original resolution of the panel. One of the methods comprises sequentially displaying full resolution left and right images. Practical embodiments of the method are disclosed in U.S. Pat. Nos. 5,465,175 and 6,014,164, FIG. 33 of U.S. Pat. No. 6,069,650, and Japanese Patent No. 2001-66547. Since each eye of the viewer periodically sees just a black screen, this method requires a very fast panel speed. That is, a low frame-rate of the imaging panel causes image flickering. In order to avoid flickering, the panel refresh rate should be about 100-120 Hz or higher. The fact that the same set of pixels of the display panel in one frame displays the left image and in the next frame displays the right image also requires a very fast panel response time. A fast response time leads to high crosstalk, i.e., right eye sees the left image and vice versa. In order to reduce crosstalk between left and right images, the image persistence should be significantly smaller than the frame period. The residual crosstalk may noticeably deteriorate perception of stereoscopic images. Since current LCD technology based on twist nematic (TN) liquid crystal does not provide short persistence, and U.S. Pat. No. 6,069,650 recommended the use of a panel based on ferro-electric liquid crystal (FLC). However, large size FLC panels are not currently in production, because of their high manufacturing cost.
Another method of displaying high resolution stereoscopic images disclosed in FIG. 8 of U.S. Pat. No. 5,606,455, U.S. Pat. No. 4,457,574, and Japanese Patent No. 2004-325494 entails sequentially displaying complimentary pairs of interlaced images with lower resolution. Two interlaced images at half of the full resolution, displayed in a sequence are perceived as a high-resolution image because of the persistence of human vision. For instance, odd columns of the left or right image can be displayed in sequence with the corresponding even columns, thus being perceived as a high resolution image. This method of displaying stereoscopic images is less sensitive to the panel's frame-rate because both left and right eyes see an image on the screen at any point in time. Interlaced images are satisfactorily viewable even with 60 Hz and 50 Hz field frequency like in national television systems committee (NTSC) and phase alternation line (PAL) television systems, respectively.
To display odd and even columns sequentially in the conventional autostereoscopic displays, left and right images should be addressed to the same set of the LCD panel. For example, to display odd columns of the left image, the left image is addressed to the odd columns of the display panel, but in the next period, to display even columns of the left image on the panel, columns are switched so that the same odd columns of the panel display the right image. However, because of the finite response time of the panel, left and right images in the odd columns cannot be exchanged immediately, which results in seeing the right images by the left eye and vice versa. This unwanted phenomenon usually called “crosstalk” noticeably deteriorates the quality of stereoscopic images.
Yet another drawback associated with the current field sequential autostereoscopic technology is the fact that graphic data used by an LCD panel is not refreshed instantly on the entire panel. It is refreshed line-by-line, which is an additional source of crosstalk if the backlight operates in a continuous wave (CW) mode, as it conventionally does.
The insufficient speed of the panel does not deteriorate the image quality as much as in the first case. That is, the insufficient speed of the panel reduces the contrast of fine details, but does not cause crosstalk between left and right images.
Recent development of fast response LCD panels has led to new opportunities for displaying stereoscopic images with full panel resolution by time multiplexing two image fields with lower resolution. To provide frame-sequential displaying, the means to produce selective viewing should be switchable between at least two states. For instance, U.S. Pat. No. 5,457,574 discloses a field sequential autostereoscopic display with switchable backlight. In the disclosed technique, a 3D display includes a directional backlight, a rear lenticular plate and a liquid crystal panel which is capable of refreshing graphic information with a refresh rate at least twice as high as is necessary for a flicker-less viewing, for example, 60-120 Hz. The backlight includes at least two separated light sources or a single light source with switchable diaphragm arranged so that the effective horizontal position of the light source can be switched synchronously by graphic data refreshing. Since the position of the viewing zone is determined by the position of the light source, each eye of the viewer can sequentially perceive the images displayed with different sets of pixels, thus seeing full resolution images. The above technique offers a solution to the problem of the displays based on the directional backlight, which is usually larger in size and more complex in manufacturing than the conventional diffuse backlight. An example of the solution using diffuse backlight is disclosed in Japanese Patent No. 2004-325494. The disclosed solution employs a flat panel, a front parallax barrier (PB) and a light deflector, placed between a viewer and the PB. The deflector periodically shifts the visual position of the display panel with PB, thus doubling the available resolution of the display in 3D mode. The disadvantage of this solution is the high cost of the wide opening deflector and significant light losses in the PB.
Another example of the solution employing diffuse backlight is disclosed in FIG. 33 of U.S. Pat. No. 6,069,650. The disclosed solution employs a switchable directional light source, which is used as a backlight for a fast LCD panel. The directional light source is composed of a diffuse backlight, an electrically addressable spatial light modulator (SLM) and a lenticular lens array.
Since left and right images in the disclosed display are displayed in a frame-sequential manner, each eye receives the sequence, which alternates between a black screen and a corresponding left or right image. To avoid flicker, the frame-rate of the display panel should be at least 100-120 Hz. Another problem with the aforementioned solution is that conventional thin film transistor (TFT) LCD panels cannot be directly applied to the display with the disclosed formation for the following reasons. A TFT LCD panel is controlled so that the left and right images sequentially inputted into the display are not actually displayed in a sequence. Images on the LCD screen are refreshed line-by-line during almost an entire frame period so that two different parts of successive images are mostly displayed on the screen at the same moment, sharing the screen as shown in FIG. 33 of U.S. Pat. No. 6,069,656. For example, an upper part of the screen displays the left image while a lower part of the screen displays the right image. FIG. 33 shows the status of the LCD panel sampled in different moments of the refresh period T. The described data refresh method could lead to severe crosstalk if the SLM switch instantly switches the polarization of the entire directional backlight. Another problem is the finite speed of the SLM switching, which also leads to crosstalk. Another source of the crosstalk is the finite switching speed of the imaging cells of the LCD panel. Since the last two kinds of crosstalk appear due to insufficient speed of switching processes, they are called dynamic crosstalk. To solve the above problems, the aforementioned display is based on an FLC type imaging panel and an FLC type SLM as the fastest LCD devices. However, FLC flat panels are currently not in production, because of their high manufacturing cost. Also, the cost of large size polarization switch based on FLC technology is too high. On the other hand, recent improvements in fast LCD panels based on TN type liquid crystal has led to new opportunities in time sequential autostereoscopic displays. It is desirable to find a way to use inexpensive imaging and switching liquid crystal panels in high-resolution autostereoscopic display. These panels are based on the use of conventional liquid crystal materials which have a switching time comparable with the frame period of LCD panel.
FIG. 1 is a simplified illustration of the technique, disclosed in FIG. 8 of U.S. Pat. No. 5,606,455. It is assumed that the backlight unit, which is not shown in the drawing, alternatively produces two sets of secondary light sources shaped as bright vertical odd lines and even lines disposed in the plane 12 and can be switched from odd lines to even lines and vice versa in synchronization with the refreshing of the LCD panel. Each line works as a lambertian light source. An LCD panel 11 is placed at a predetermined distance from the light sources to provide stereoscopic viewing zones from the viewing plane 13.
In a certain period, the LCD panel 11 displays odd columns of the left and right images registered alternatively, denoted by the letters R and L in FIG. 1A. The LCD cells modulate the light coming from odd light sources (even lines are “off”) so that the right eye of a viewer receives only the light passed through the right image columns and the left eye receives only the light passed through the left image columns, thus producing a stereoscopic sensation. In a next period (see FIG. 1B), the columns, which formerly displayed the right image, are activated to display the left image and vice versa. This new graphic data represents even columns of the left and right half-resolution images. The light source is also switched so that the even lines are on and odd lines are “off”. A viewer sees the light from even lines, modulated by the left and right columns and the position of the lines is interlaced with their position from the previous frame. Due to the persistence of human vision, odd and even half-resolution fields, shown sequentially, are perceived as one full-resolution stereoscopic image
Because of the finite response time of the TN LCD panel, left and right images in the columns cannot be exchanged immediately, resulting in seeing the right images by the left eye, and vise versa. This unwanted phenomenon called crosstalk noticeably deteriorates the quality of stereoscopic images.
Yet another drawback associated with the aforementioned technique is the fact that graphic data on an LCD panel is not refreshed instantly on the entire panel. It is refreshed line-by-line, which is an additional source of crosstalk if the backlight operates in a CW mode, as it conventionally does.